Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session L26: Superconducting Qubits: Noise and Decoherence IIFocus

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Sponsoring Units: DQI Chair: Britton Plourde, Syracuse Univ Room: BCEC 160B 
Wednesday, March 6, 2019 11:15AM  11:27AM 
L26.00001: Correlating decoherence in transmon qubits:
Low frequency noise by single fluctuators Steffen Schlör, Jürgen Lisenfeld, Clemens Müller, Andre Schneider, Alexey Ustinov, Martin Weides We report on longterm measurements of a highcoherent, nontunable transmon qubit, revealing low frequency burst noise in coherence and transition frequency. We present a simultaneous measurement of the qubit's relaxation and dephasing rates as well as resonance frequency fluctuations, and analyze their correlations. These yield information about the microscopic origin of the intrinsic decoherence mechanisms in Josephson qubits and their fluctuation dynamics. From a spectral noise analysis we obtain further evidence for our presented model of a small number of dominant fluctuators. 
Wednesday, March 6, 2019 11:27AM  11:39AM 
L26.00002: Correlation of Lifetime Fluctuations in Superconducting Qubits Dario Rosenstock, Joshua Carey, Chen Wang While the energy relaxation times (T_{1}) of superconducting qubits have improved greatly since the birth of the field, much work remains to better understand the limitations on lifetimes and how best to extend them. It is widely observed that qubits exhibit timedependent fluctuations of their T_{1} times, but the main sources of this process remain a mystery. Among the leading candidates are spurious resonant twolevel systems in the dielectric surrounding the device, which are frequencyspecific, and excess quasiparticles near the junction, which are not. We monitor the T_{1} fluctuations for the first two excited states of 3D fluxtunable transmon and fluxonium qubits and examine correlations between the decay rates. We measure T_{1} of the e> state at a flux bias such that the g>  e> transition frequency matches that of the unbiased e>  f> transition. This allows us to resolve frequency dependence of the T_{1} fluctuations. We believe this is a helpful general tool to distinguish between the effects of dielectric and quasiparticle loss across a range of devices. 
Wednesday, March 6, 2019 11:39AM  11:51AM 
L26.00003: Crosscorrelation noise measurements of a graphenebased SQUID magnetometer Jonathan Prance, Michael Thompson, Richard Haley, Yuri Pashkin, Moshe Ben Shalom, Vladimir Falko, Harriet van der Vliet, Anthony Matthews, Ziad Melhem Lateral superconductor/graphene structures can be used to make Josephson junctions with low contact resistances and gatetuneable critical currents [1]. These junctions have the potential to provide new functionality for superconducting devices. For most devices, e.g. transmon qubits and SQUID sensors [2], it is important to quantify the intrinsic noise of the junctions. The voltage noise of low resistance junctions is typically below the noise floor of room temperature amplifiers. By crosscorrelating the signals from two parallel amplifiers, we can detect signals down to ~100 pV/√Hz, well below the noise floor of each amplifier. Using this technique, we characterise the voltage noise of a NbTi DC SQUID with graphene junctions in a frequency range from ~Hz to ~kHz. Combined with measurements of the SQUID’s gain, we map its sensitivity across a range of operating conditions and find that the bestcase sensitivity of the device is similar to traditional low temperature SQUIDs with oxide tunnel junctions. 
Wednesday, March 6, 2019 11:51AM  12:03PM 
L26.00004: How to reduce energy loss at the interface of superconducting devices Igor Diniz, Rogério de Sousa We present a quantum theory of dielectric energy loss due to the interaction of photons, tunneling two level systems (TTLSs), and phonons in superconducting devices. As each TTLS couple to photons and phonons, it induces a photonphonon interaction that can be described as an effective piezoelectric effect. We show that most energy loss occurs at the interface, with phonon interference playing an important role, leading to several predictions that can be tested in current experiments. Explicit numerical calculations of the loss tangent in devices demonstrates that dielectric energy loss can be reduced by exploiting destructive interference of the phonon radiation emitted from different interfaces. 
Wednesday, March 6, 2019 12:03PM  12:15PM 
L26.00005: MagnetoElectric Coupling of Noise and LossGenerating Paramagnetic Spins in Superconducting Circuits Keith Ray, Jonathan L DuBois, Vincenzo Lordi Noise and loss in superconducting circuits caused by fluctuating charge and magnetic flux represent significant challenges for the realization of largescale quantum computing architectures. Previous experimental [Phys. Rev. Applied 6 041001 (2016), Phys. Rev. Lett. 118, 057703 (2017)] and theoretical [PRL 112 017001 (2014)] work has identified O2, OH, and atomic H surface adsorbates as possible sources of flux noise in superconducting circuits. We report here on an extension to our model for the flux noise generated by the dynamics of an ensemble of paramagnetic spins on adsorbed O2 molecules to include electric charge density differences associated with spin flips. This model combines a thermodynamic ensemble generated with Monte Carlo simulations with LandauLifshitzGilbert equation simulations for the dynamics and is parametrized with vdWcorrected density functional theory calculations. From this model we evaluate the effects of external electric and magnetic fields on the phases of the spin system, its dynamics, and the charge and flux noise generated. 
Wednesday, March 6, 2019 12:15PM  12:27PM 
L26.00006: NonGaussian Noise Spectroscopy with a Superconducting Qubit Youngkyu Sung, Felix Beaudoin, Leigh Norris, Fei Yan, David K Kim, Jack Yanjie Qiu, Uwe Von Luepke, Jonilyn L Yoder, Terry Philip Orlando, Lorenza Viola, Simon Gustavsson, William D Oliver Most quantum control and quantum errorcorrection protocols assume that the noise causing decoherence is described by Gaussian statistics. However, the Gaussianity assumption breaks down when a qubit is strongly coupled to a sparse environment or has a nonlinear response to environmental degrees of freedom. Here, we experimentally validate an openloop quantum control protocol that reconstructs the higherorder spectrum of injected nonGaussian phase noise using a superconducting qubit as a noise spectrometer. This first experimental demonstration of nonGaussiannoise spectroscopy represents a major step toward the goal of demonstrating a complete noise spectral characterization of quantum devices. 
Wednesday, March 6, 2019 12:27PM  12:39PM 
L26.00007: Sources of decoherence in fixed frequency transmon qubits. Andreas Fuhrer, Matthias Mergenthaler, Peter Mueller, Stephan Paredes, Clemens Müller, Marc Ganzhorn, Stefan Filipp, Thilo Stoeferle, Gian Salis Significant advances in the coherence of superconducting qubits were made possible by clever microwave engineering. For the transmon qubit this was mainly the reduction of its charge dispersion by capacitively shunting the Josephson junction, the increased size of its capacitor pads, filtering of the microwave controls and operation at noise insensitive points. More recently, efforts were made to disentangle the contributions of various material interfaces by engineering participation ratios of the electromagnetic field with different geometries of resonators and qubits in order to pinpoint the sources of noise. 
Wednesday, March 6, 2019 12:39PM  12:51PM 
L26.00008: TLS induced decoherence instabilities in superconducting qubits Andreas Bengtsson, Jonathan Burnett, Marco Scigliuzzo, David Niepce, Marina Kudra, Per Delsing, Jonas Bylander We study the temporal stability of relaxation and dephasing in superconducting transmon qubits. By collecting statistics during measurements spanning multiple days, we reveal large fluctuations of qubit lifetimes and find that the main cause of T_{1} fluctuations is interacting parasitic twolevelsystems (TLS). Our statistical analysis also provides useful information about dynamics of TLS which could help identify possible microscopic sources of TLS, which still remain elusive. Moreover, interacting TLS also cause capacitance fluctuations, ultimately leading to frequency noise and dephasing of the qubit state. These discoveries are important for manufacturing stable superconducting circuits suitable as a scalable quantum computing platform where drift and fluctuations lead to unnecessary calibration and downtime. 
Wednesday, March 6, 2019 12:51PM  1:03PM 
L26.00009: Charge noise, fluxonium, and all that Ari M Mizel, Yariv Yanay Fluxonium qubits were designed to eliminate charge offsets. We consider the theory of fluxonium qubits and distill crucial features that reduce vulnerability to charge noise. Our observations prompt a new superconducting qubit design with some attractive features. 
Wednesday, March 6, 2019 1:03PM  1:15PM 
L26.00010: Simulations of Magnetic Noise in Classical XY and Heisenberg Spin Models Daniel Mickelsen, Hui Wang, Zhe Wang, Ruqian Wu, Clare C Yu Superconducting qubits show great promise but continue to be plagued by flux noise. Experiments show that surface spins are the source of this flux noise, and the noise has a power spectral density of the form 1/f^{α} with the noise exponent α~1. Experiments also provide evidence for ferromagnetic exchange. We investigate to what extent ferromagnetic exchange is consistent the observed flux noise exponents. With input from density functional theory calculations of magnetic impurities, we present the results of Monte Carlo simulations of the magnetic noise produced by coupled classical XY and Heisenberg spins in 2D lattices. We find the parameters of the models that result in a noise exponent near 1. 
Wednesday, March 6, 2019 1:15PM  1:27PM 
L26.00011: Experimental study of flux noise in nanowire transmons subject to an applied magnetic field Thijs Stavenga, Florian Luthi, Joep Assendelft, David Thoen, Akira Endo, Peter Krogstrup, Leonardo DiCarlo Flux noise is generally the dominant dephasing mechanism in transmon qubits using SQUID loops as a tunable inductive element. Continuing experimental research ambitions to elucidate the microscopic origin and mitigation of flux noise. In this work, we experimentally investigate flux noise using an unconventional knob for circuit QED: an applied inplane magnetic field. We first present various changes introduced to our circuit QED system and setup to further extend the field compatibility of resonators and nanowire transmons beyond 70 mT and to reduce extrinsic noise. Next, we use standard coherence measurements and sensitivity analysis on and off flux sweetspots to investigate the dependence of flux noise on field applied along the axis of the two constituent nanowire junctions. 
Wednesday, March 6, 2019 1:27PM  1:39PM 
L26.00012: Measuring Quantum Noise Limits in Superconducting Digital Circuits Aaron Lee, Micah Stoutimore, John X Przybysz, Aaron Pesetski, Oliver Oberg, Nathan Mungo, James Medford, Lewis Graninger A fundamental question in Josephson junction physics is the temperature at which quantum tunneling becomes the dominant source of junction escape as opposed to thermal excitation. This quantum crossover temperature is important to the design of classical superconducting circuits, as it dictates the minimum error rate that can be achieved for a given junction size. This property has been previously described and measured for over forty years, but the measurement technique is limited to single junctions and not extensible to junctions embedded in a larger circuit. Here we demonstrate a technique for measuring the quantum crossover temperature for a superconducting digital circuit by examining the width of the transition from operation to failure in a basic Reciprocal Quantum Logic digital circuit. This transition width is extracted from the broadband noise generated from the circuit errors and further, disambiguates from spectral noise, such as line noise, that can broaden this transition and artificially increase the crossover temperature. Application of this technique to a medium sized circuit demonstrates that the quantum crossover temperature of a junction embedded in a larger circuit is indistinguishable from the crossover temperature extracted from an isolated junction. 
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